The performance of semiconductor devices, specifically metal oxide semiconductor field effect transistor (MOSFET) devices, may be enhanced by using a strained silicon layer to accommodate the device channel region (introducing a strain along the length of the channel). This layer allows the device channel region to experience increased carrier mobility which increases device performance. This carrier transport enhancement mechanism through the utilization of strained silicon is relatively new.
One method of forming a strained silicon layer is to grow an epitaxial layer (or epilayer) of a semiconductor material having a first natural lattice constant on top of a substrate having a second natural lattice constant different from the first. A biaxially strained epitaxial layer of the overlying semiconductor material may thereby be formed. The term natural lattice constant is defined as the lattice constant of the bulk, unstrained crystalline material.
In one example, the epitaxial layer may be formed from silicon, and the substrate may be formed from a silicon germanium (SiGe) alloy or layer. Silicon has a natural lattice constant of approximately 5.43 Angstroms. Silicon germanium has a natural lattice constant of between 5.43 and 5.66 Angstroms, depending upon the concentration of germanium in the SiGe. The higher the concentration of germanium, the larger the natural lattice constant of the SiGe. With the natural lattice constant of SiGe is higher than that of silicon, the entire silicon epilayer will be in a state of biaxial tensile stress. U.S. Pat. No. 6,867,428 to Besser, et. al., incorporated herein by reference, discloses a strained silicon nFET having a strained silicon channel formed in such an epilayer.
However, the biaxial tensile channel is normally achieved by a complex and costly process flow. Since high Ge concentration in the relaxed SiGe layer is required in order to have both electron and hole enhancement, an underlying thick virtual substrate with a gradual increase or increment of Ge concentration is needed before forming the relaxed SiGe layer.
One prior art method of constructing these layers is described in U.S. Pat. No. 6,593,641 to Fitzgerald, which is incorporated herein by reference. Fitzgerald teaches a compositionally graded layer of SiGe having a relax SiGe layer formed thereon. Fitzgerald teaches forming the composition graded SiGe layer on a first silicon substrate, with a relax SiGe layer formed thereon. The relaxed SiGe layer is then “transferred” to a second silicon substrate using conventional bonding. The composition graded SiGe layer and the first silicon substrate are then removed, resulting in the second silicon substrate with relaxed SiGe layer. Fitzgerald alternatively teaches an SiGe on insulator substrate (termed an “SGOI”) where the second substrate may be coated with silicon dioxide before bonding to the relax SiGe layer, or both wafers can be coated with silicon dioxide to enable oxide-to-oxide bonding. This structure is now commonly referred to as a Strain-Si-Direct-On-Insulator (SSDOI) structure.
As described in Fitzgerald, and similar to SOI formation, two semiconductor wafers are needed to form a single SSDOI wafer. The wafers are bonded followed by separation at a predetermined location (the relax SiGe layer) to form one SSDOI wafer. In addition, formation of the relatively thick composition-graded or buffer SiGe layer is time-consuming and complicated.
Therefore, the prior art processing techniques for forming SSDOI wafers are complex, costly and time-consuming because two wafers are required in order to form a single SSDOI substrate/wafer.
Accordingly, there are needed improved fabrication processes and semiconductor wafer/substrates that reduce cost and increases throughput improvement over the existing processes. In general terms, the present disclosure describes processes and semiconductor wafer structure(s) that enable the formation of (n−1) SSDOI wafers by using the n wafers.